ABU DHABI, United Arab Emirates — The United States now believes "with varying degrees of confidence" that the Syrian government has used chemical weapons against its people, according to Defense Secretary Chuck Hagel and the White House.

Hagel, speaking to reporters in Abu Dhabi, also said the U.S. government believes "any use of chemical weapons in Syria very likely originated" with President Bashar Assad. These announcements are the first U.S. indication of confidence in claims of Syria's chemical weapons use, and represent a reversal for the government.

President Obama has said the use of chemical weapons by Syria, or the transfer of stockpiles to terrorist groups, would be a "red line" that could trigger U.S. reaction.

But White House and Pentagon officials caution more proof is needed before the United States escalates its involvement in Syria, particularly the use of any military force aimed at ending a bloody civil war that has claimed thousands of lives or ousting Assad's regime.

These officials, who spoke on condition of anonymity, cited faulty intelligence about the use of weapons of mass destruction such as in Iraq -- which proved to be wrong -- as a reason for the careful approach.

"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," White House legislative director Miguel Rodriguez wrote in letters to Sen. Carl Levin, D-Mich., and John McCain, R-Ariz.

Hagel said the decision to conclude chemical weapons use by the Syrian government was "reached in the last 24 hours."

"The intelligence community has been assessing information for some time on this issue," he said, adding that intelligence is still being analyzed to determine when and under what circumstances the weapons were used.

Any use of chemical weapons by Assad's forces "violates every convention of warfare," Hagel said, since they represent "uncontrollable, deadly weapons."

A White House official said the United States will be "methodical, rigorous and relentless" as it seeks more definitive proof about Syria's chemical weapons use. The official did not give a timeline for action.

If evidence definitively shows the Assad regime has used chemical weapons, the official said, the U.S. will consult with its global allies and decide on the next steps. "All options are on the table," said the official, who was not authorized to speak publicly.

The assessment about Syria's chemical weapons use sparked a sharp reaction on Capitol Hill, where Republican and Democratic senators cited it as evidence that Obama's "red line" had been breached.

"It is clear that 'red lines' have been crossed and action must be taken to prevent larger scale use," said Senate Intelligence Committee Chairwoman Dianne Feinstein, D-Calif. She urged the United Nations Security Council to take "strong and meaningful action" against Syria.

McCain and Sen. Lindsey Graham, R-S.C., emerging from a national security briefing, said the chemical weapons stocks must be secured and aid should be provided to the Syrian Opposition Council to drive out the Assad regime.

"I think it's pretty obvious that a red line has been crossed," McCain said. "Now I hope the administration will consider what we have been recommending now for over two years of this bloodletting and massacre, and that is to provide a safe area for the opposition to operate, to establish a no-fly zone and provide weapons to the people in the resistance who we trust."

In his letter, Rodriguez wrote that "our standard of evidence must build on these intelligence assessments as we seek to establish credible and corroborated facts. For example, the chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions."

The assessment of weapons use relies in part on "physiological evidence," a senior defense official said, declining to be more specific.

Earlier this week, Hagel said he doubted Syria had used the weapons after an Israeli intelligence officer had said so during a security conference in Israel.

Brig. Gen. Itai Brun, the head of research and analysis in Israeli military intelligence, said Tuesday that Syria had used chemical weapons multiple times. "Shrunken pupils, foaming at the mouth and other signs indicate, in our view, that lethal chemical weapons were used," he said.

Brun also said the Syrian regime was using less lethal chemical weapons, and that Russia has continued to arm the Syrian military with weapons such as advanced SA-17 air defense missiles.

The White House statement puts the United States closer to the positions of Great Britain, France and Israel about Syria's use of chemical weapons. British and French officials told U.N. Secretary General Ban Ki Moon last month they had evidence of such use, including soil samples and interviews with witnesses and members of the Syrian opposition.

The two countries asked the United Nations chief to investigate allegations of chemical weapons use in two locations near Damascus on March 19, as well as in the city of Homs on Dec. 23. Ban has appointed a team of chemical weapons experts to investigate the allegations, but the Syrian government has largely blocked the team from doing its work. Syria, meanwhile, has accused rebels of using chemical weapons.

Rodriguez wrote that the United States is "currently pressing for a comprehensive United Nations investigation that can credibly evaluate the evidence and establish what took place."

Feinstein, the Senate intelligence chairwoman, called on the U.N. Security Council to act against Syria.

"I am very concerned that with this public acknowledgement, President Assad may calculate he has nothing more to lose and the likelihood he will further escalate this conflict therefore increases," Feinstein said in a statement. "It is also important that the world understands the use of weapons of mass destruction, such as sarin, will not be countenanced, and clearly Assad must go."

Rep. Adam Smith of Washington, the top Democrat on the House Armed Services Committee, urged discretion during an appearance on C-SPAN's Newsmakers program.

Smith said the United States should talk to its allies before committing to any action. "Personally I urge caution," he said, adding that he does not want to commit U.S. troops to Syria. "We do not need another war."

"What has not been said by the administration at any point beyond the red line is what that means," he said. "What will they do? What is the best response?"